2-Hydroxypropionic acid, that is, lactic acid, is widely applied not only to uses such as food and pharmaceuticals, but also to industrial uses as a monomer material for plastics, and has been increasingly demanded. Lactic acid is known to be produced by, in addition to chemical synthesis, fermentation by microorganisms, wherein the microorganisms convert substrates containing hydrocarbons such as glucose into lactic acid. Lactic acid is divided into optical isomers, the (L)-isomer and the (D)-isomer based on the conformation of the substituent bound to the carbon at the α-position of carbonyl. By appropriately selecting the microorganism for microbial fermentation, (L)- or (D)-lactic acid can be selectively produced, or lactic acid as a mixture of the (L)-isomer and the (D)-isomer (racemic body) can be produced.
Production of lactic acid by microbial fermentation is generally carried out while a pH appropriate for the microbial fermentation is maintained by addition of an alkaline substance (e.g., calcium hydroxide) to the culture broth. Most of the lactic acid produced as an acidic substance by the microbial fermentation is present as a lactic acid salt (e.g., calcium lactate) in the culture broth due to the addition of the alkaline substance. In general, when lactic acid is used as monomers for a plastic, the lactic acid is preferably lactic acid in the free form obtained by adding an acidic substance (e.g., sulfuric acid) to the culture broth after completion of the fermentation. However, the lactic acid fermentation broth obtained by microbial fermentation contains, other than lactic acid as the product of interest, organic acids and salts thereof, proteins, amino acids, and nonionic compounds such as glycerol, as impurities. When the lactic acid is used as monomers for a plastic, the lactic acid needs to be separated from these impurities.
As a method of removing impurities from a lactic acid-containing aqueous solution and recovering lactic acid, a method based on crystallization of lactic acid is known. In the crystallization, lactic acid is precipitated as crystals to increase the chemical purity as well as the optical purity of the lactic acid, and most of impurities are distributed into the liquid component (mother liquor). As an example of purification of lactic acid by crystallization, Japanese Translated PCT Patent Application Laid-open No. 2002-540090 discloses a method in which an aqueous lactic acid solution is distilled, and crystallization is then performed to recover high-quality lactic acid. WO 2009/004922 discloses a method in which a lactic acid-containing solution derived from microbial fermentation is passed through a nanofiltration membrane, and crystallization of lactic acid is then performed to increase the yield.
We found a problem in the conventional methods that, when lactic acid crystals are obtained by crystallization from a lactic acid-containing solution and the obtained lactic acid crystals are recovered by solid-liquid separation, the recovered lactic acid crystals cannot be easily separated from the liquid so that the purity of the lactic acid is low.
It could therefore be helpful to provide, when lactic acid crystals are obtained by crystallization from a lactic acid-containing solution and the obtained lactic acid crystals are recovered by solid-liquid separation, a method of obtaining lactic acid crystals exhibiting excellent separation from the liquid (solid-liquid separation performance).